Measuring and modelling VOC emissions from grassland

Volatile organic compounds (VOC) are emitted through anthropogenic activities (from landfills and fossil fuel combustion) and from animals, but the main source are plants. The most prevalent non methane biogenic VOC are: isoprene, mono- and sesquiterpenes, as well as oxygen containing compounds such as methyl butenol and other alcohols, carbonyls and organic acids. The emission of VOC strongly affects the chemical and physical properties of the atmosphere, for example VOC react to form tropospheric ozone, which itself is a greenhouse gas and indirectly reinforces global warming by decreasing the concentration of the hydroxyl radical, which in turn affects the lifetime and concentration of radiatively active trace gases such as methane in the atmosphere. Despite their importance for atmospheric chemistry, there have been relatively few studies quantifying the long term ecosystem scale emission of a larger number of VOC. Accordingly, global VOC emission estimates are highly uncertain. The objectives of the proposed project are threefold: The first objective is to quantify the range of VOC emitted from mountain grasslands and their emission strength during the full vegetative period. We hypothesise that, in addition to strong emissions of methanol, acetaldehyde, hexenals and acetone following grass cutting, grasslands will exhibit relatively high background emissions of methanol, as well as significant background fluxes of ethanol, acetaldehyde and acetone. To this end field measurements of VOC, carbon dioxide (CO2) and energy fluxes will be conducted above a meadow in Tyrol (Austria) over two full vegetation periods using the eddy covariance method. The second objective is to test the applicability of a novel PTR TOF MS (time-of-flight proton-transfer mass spectrometer) for measuring turbulent VOC fluxes between grassland and the atmosphere. To this end in addition to a conventional PTR MS the novel PTR TOF MS will be operated in the field during short intensive measuring campaigns for comparison. The third objective, finally, is to include a VOC emission module based on combined physiological and physicochemical control into an existing soil-vegetation-atmosphere-transfer (SVAT) model and to validate and test it using the flux measurements described above. The new model will be parameterised by means of Bayesian calibration against measured VOC, CO2 and energy fluxes and then used to study the controls on VOC emission as well as their relation to CO2 and energy fluxes.

Land ecosystems represent a significant biogenic source of volatile organic compounds (VOC) that by far exceeds human VOC sources. VOCs play a pivotal role in atmospheric chemistry and land ecosystems thus have the ability to modulate regional air chemistry and global climate. The objectives of the project were threefold: (1) To quantify the range of VOC emitted from mountain grasslands and their source/sink strength during the full vegetative period, (2) to test the applicability of the PTR-TOF for measuring turbulent VOC fluxes between grassland and the atmosphere, and (3) to parameterize, validate and test a process-based VOC gas exchange model. VOC flux measurements were carried out during two full vegetation periods (2008 and 2009) using a PTR-MS and the virtual disjunct eddy covariance method. We found that only few compounds are exchanged during the entire growing period. The quantitatively most important compound was methanol, followed by acetaldehyde and acetone. The investigated grassland was a source for these three compounds except for acetone, for which it was a sink in 2009. During and after cutting of the vegetation and the application of organic manure large departures in the composition and magnitude of VOC exchange were observed. After a hail storm in 2009, which severed many of the conifer trees found on the mountain slopes surrounding the study site and triggered large monoterpene and to a lesser extend sesquiterpene emissions into the atmosphere, we observed for the first time ever sustained, significant deposition of these terpeneoids to the investigated grassland. The new PTR-TOF instrument was extensively tested and cross-compared to the PTR-MS for making eddy covariance VOC flux measurements. It could be shown that the eddy covariance fluxes measured with the PTR-TOF agree very well with the PTR-MS. The huge advantage of the new PTR-TOF instrument is the fact that full mass scans (up to m/z 315) are available at a much higher frequency (e.g. at 10Hz). This reduces the random variability of the flux calculations and offers a huge potential for detecting fluxes of compounds that have been previously overlooked. In contrast to what was expected, first modelling studies indicated that the intended process-oriented modelling approach did not work very well. Rather simpler, empirical models were more successful in simulating VOC fluxes at ecosystem-scale observed in the field. The reasons for this are not clear at present. Most likely it is a combination of mismatch in scale between leaf-level studies, from which most published models derive, and ecosystem-scale flux measurements (which include the soil as a sink/source of VOC), as well between controlled (and sometimes artificial) environmental conditions in lab experiments and real-world environmental conditions in the field.